Hi Paul, there’s a fairly active HPN Facebook page in case you’re not aware - these occasionally come up there and I got a good response when I asked the same question.
You’re aware of the two variants I’m sure, others may not be - screw filler cap = PTFE and not paintable (well, not if you want it to stay on for any significant length of time); Bayonet filler cap = Nylon and paintable.
Alternatively Richie Moore can still get these new via HPN.